Ferromagnetism in manganese compound semiconductors not only opens prospects for tailoring magnetic and spin-related phenomena in semiconductors with a precision specific to III-V compounds but also addresses a question about the origin of the magnetic interactions that lead to a Curie temperature (T(C)) as high as 110 K for a manganese concentration of just 5%. Zener's model of ferromagnetism, originally proposed for transition metals in 1950, can explain T(C) of Ga(1-)(x)Mn(x)As and that of its II-VI counterpart Zn(1-)(x)Mn(x)Te and is used to predict materials with T(C) exceeding room temperature, an important step toward semiconductor electronics that use both charge and spin.

A mean field model of ferromagnetism mediated by delocalized or weakly localized holes in zinc-blende and wurzite diluted magnetic semiconductors is presented. The model takes into account: (i) strong spin-orbit and kp couplings in the valence band; (ii) the effect of strain upon the hole density-of-states, and (iii) the influence of disorder and carrier-carrier interactions, particularly near the metal-to-insulator transition. A quantitative comparison between experimental and theoretical results for (Ga,Mn)As demonstrates that theory describes the values of the Curie temperatures observed in the studied systems as well as explain the directions of the easy axis and the magnitudes of the corresponding anisotropy fields as a function of biaxial strain. Furthermore, the model reproduces unusual sign, magnitude, and temperature dependence of magnetic circular dichroism in the spectral region of the fundamental absorption edge. Chemical trends and various suggestions concerning design of novel ferromagnetic semiconductor systems are described.Comment: 25 pages, 16 figure

It is often assumed that it is not possible to alter the properties of magnetic materials once they have been prepared and put into use. For example, although magnetic materials are used in information technology to store trillions of bits (in the form of magnetization directions established by applying external magnetic fields), the properties of the magnetic medium itself remain unchanged on magnetization reversal. The ability to externally control the properties of magnetic materials would be highly desirable from fundamental and technological viewpoints, particularly in view of recent developments in magnetoelectronics and spintronics. In semiconductors, the conductivity can be varied by applying an electric field, but the electrical manipulation of magnetism has proved elusive. Here we demonstrate electric-field control of ferromagnetism in a thin-film semiconducting alloy, using an insulating-gate field-effect transistor structure. By applying electric fields, we are able to vary isothermally and reversibly the transition temperature of hole-induced ferromagnetism.

Recent works aiming at understanding magnetotransport phenomena in ferromagnetic III-V and II-VI semiconductors are described. Theory of the anomalous Hall effect in p-type magnetic semiconductors is discussed, and the relative role of side-jump and skew-scattering mechanisms assessed for (Ga,Mn)As and (Zn,Mn)Te. It is emphasized that magnetotransport studies of ferromagnetic semiconductors in high magnetic fields make it possible to separate the contributions of the ordinary and anomalous Hall effects, to evaluate the role of the spins in carrier scattering and localization as well as to determine the participation ratio of the ferromagnetic phase near the metal-insulator transition. A sizable negative magnetoresistance in the regime of strong magnetic fields is assigned to the weak localization effect.

This review compiles results of experimental and theoretical studies on thin films and quantum structures of semiconductors with randomly distributed Mn ions, which exhibit spintronic functionalities associated with collective ferromagnetic spin ordering. Properties of p-type Mncontaining III-V as well as II-VI, IV-VI, V 2 -VI 3 , I-II-V, and elemental group IV semiconductors are described paying particular attention to the most thoroughly investigated system (Ga,Mn)As that supports the hole-mediated ferromagnetic order up to 190 K for the net concentration of Mn spins below 10%. Multilayer structures showing efficient spin injection and spin-related magnetotransport properties as well as enabling magnetization manipulation by strain, light, electric fields, and spin currents are presented together with their impact on metal spintronics. The challenging interplay between magnetic and electronic properties in topologically trivial and nontrivial systems is described, emphasizing the entangled roles of disorder and correlation at the carrier localization boundary. Finally, the case of dilute magnetic insulators is considered, such as (Ga,Mn)N, where low temperature spin ordering is driven by short-ranged superexchange that is ferromagnetic for certain charge states of magnetic impurities.

Over the past ten years, the search for compounds combining the properties of semiconductors and ferromagnets has evolved into an important field of materials science. This endeavour has been fuelled by many demonstrations of remarkable low-temperature functionalities in the ferromagnetic structures (Ga,Mn)As and p-(Cd,Mn)Te, and related compounds, and by the theoretical prediction that magnetically doped, p-type nitride and oxide semiconductors might support ferromagnetism mediated by valence-band holes to above room temperature. Indeed, ferromagnetic signatures persisting at high temperatures have been detected in a number of non-metallic systems, even under conditions in which the presence of spin ordering was not originally anticipated. Here I review recent experimental and theoretical developments, emphasizing that they not only disentangle many controversies and puzzles accumulated over the past decade but also offer new research prospects.

No abstract

The presence of a ferromagnetic transition in single, modulation-doped, 8 nm quantum well of Cd 0.976 Mn 0.024 Te͞Cd 0.66 Mg 0.27 Zn 0.07 Te:N is evidenced by photoluminescence magnetospectroscopy. The transition is driven by long range Ruderman-Kittel-Kasuya-Yosida interactions between Mn spins, mediated by 2 3 10 11 holes per cm 2 . It occurs at 1.8 K, in agreement with a mean-field model. [S0031-9007(97)03602-8] PACS numbers: 75.70.Cn, 75.30.Hx, 75.50.Pp, 78.55.Et Because of complementary properties of semiconductor and ferromagnetic material systems, a growing effort is directed toward studies of hybrid semiconductor-magnetic nanostructures. Such devices, in which both electric and magnetic fields are spatially modulated, have usually been fabricated by patterning a ferromagnetic metal on the top of a modulation-doped GaAs/AlGaAs heterostructure [1] or by incorporation of magnetic clusters directly into a semiconductor matrice [2].In this Letter, we show that the two-dimensional hole gas confined in modulation-doped quantum wells of Cd 12x Mn x Te produces, via the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism, a ferromagnetic coupling between the Mn spins. Actually, by the direct observation of a ferromagnetic phase transition, we demonstrate that this coupling can overcompensate antiferromagnetic interactions specific to II-VI diluted magnetic semiconductors (DMS) [3]. Our results mean, therefore, that the well-established methods of modulation of the carrier concentration in semiconductor quantum structures can be applied for tailoring of the magnetic properties. The transition to the ferromagnetic phase is put into the evidence by observing colossal Zeeman splittings of interband optical transitions, probed here by means of photoluminescence (PL) and its excitation spectra (PLE), a technique equivalent to absorption spectroscopy but which can be used with a strongly absorbing substrate. A quantitative description of our findings confirms predictions of a recent model [4] on the free carrier-induced ferromagnetism in structures of doped DMS. It makes it also possible to evaluate the strength of many body effects for the case of two-dimensional hole gas. Moreover, the data provide important information on critical phenomena in the disordered magnetic systems of reduced dimensionality, illustrating, in particular, how long-range spin-spin interactions stabilize an ordered phase and make fluctuations of magnetization irrelevant.Our studies have been carried out on samples grown in a molecular beam epitaxy (MBE) chamber equipped with a home-designed electron cyclotron resonance (ECR) plasma cell as a nitrogen source. Prior to fabrication of the proper structures, doping characteristics of the barrier material Cd 12y2z Mg y Zn z Te have been determined by means of the Hall effect, capacitance-voltage profiles, cathodoluminescence, and x-ray diffraction. It has been found [5] that by lowering the growth temperature down to 220-240 ± C it becomes possible to reduce the nitrogen-induced diffusion of Mg atom...